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Lady Hobbs. Do you think that 21 fish in a 30 gallon tank is a little on the high side for this capacity fish tank. I know this depends on tank filtration and maintenance as well. Cardinals are fairly sensitive overall. It could be more then one issue leading to death. It could be multiple issues like: stress, not enough oxygen, nitrates, high mineral content, inadequate filtration. Like I said cardinals are a little sensitive